(1) Field of the Invention
This invention relates to a hydrogen-absorbing alloy electrode used as a negative electrode of an alkaline storage cell.
(2) Description of the Prior Art
An alkaline storage cell employing a hydrogen-absorbing alloy for its negative electrode has the following advantages over a nickel-cadmium cell.
(a) Can have higher energy density. PA1 (b) Can have longer cycle life. PA1 (c) Can have excellent over discharge characteristics. PA1 (d) Can endure fast charge.
Because of these advantages, a hydrogen-absorbing alloy has recently attracted attention as a material of the next-generation alkaline storage cell and been thoroughly researched on.
Especially active research is taking place on hydrogen-absorbing alloys which contain rare earth elements. These type of alloys have CaCu.sub.5 -type hexagonal structure such as LaNi.sub.5. Since cells employing the above alloys are excellent in (a) and (b), various improvements in various aspects have been tried on such cells. One of the improvements is to use Misch metal (Mm) instead of expensive La in order to reduce manufacturing cost of the alloy. Mm is a mixture of rare earth elements.
Japanese Patent Publication laid-open No. 62-20245 has disclosed an example of such an improvement, which is hydrogen-absorbing alloys represented by a formula LnNi.sub.x (Co.sub.a Mm.sub.b Al.sub.c).sub.y [Ln is Mm only or a mixture of Mm and La; La is contained in Mn in a weight ratio (%) of 25.ltoreq.La.ltoreq.75 the other parameters are 3.5.ltoreq.x.ltoreq.4.3; 0.7.ltoreq.y.ltoreq.1.7; 4.3.ltoreq.x+y.ltoreq.5.5; 0.2.ltoreq.b.ltoreq.0.8; and 0.1.ltoreq.c.ltoreq.0.5.]
However, negative electrodes employing the above alloys are not desirable for use in a sealed type storage cell in terms of safety because hydrogen absorption/desorption equilibrium pressures of the alloys are high. Neither do they achieve much improvement in cell characteristics such as cycle characteristics.